It is known in the art to produce diffractive optical elements which manipulate incident light beams, an example light beam manipulation being the conversion of planar or spherical wavefronts to generalized wavefronts. The diffractive optical elements are generally thinner, lighter, can be corrected for many types of aberrations and distortions and can combine several functions into one element. They are generally more versatile than standard optical elements and are, therefore, desirable for use laser beam handling systems such as laser scanners, compact discs, laser computerized processing, laser radars and bar code scanners.
The article, "Micro Fresnel Lenses" by H. Nishihara and T. Suhara in Progress in Optics XXIV, edited by E. Wolf, presents background information on diffraction lenses of the Fresnel type.
Computed generated diffractive optical elements are attractive since they may be designed to perform very complex operations on the phase of the incident light beams.
A computer generated diffractive optical element with high efficiency is the Kinoform, described in the article "The Kinoform: A New Wavefront Reconstruction Device," by L. B. Lesem, P. M. Hirsch, and J. A. Jordan published in The IBM Journal of Research and Development, Vol. 13, pp. 150-155, 1960. However, since the Kinoform has a continuous phase profile which must be accurately produced, it is difficult to manufacture.
As is discussed in the article "Blazed Synthetic Phase-Only Holograms," by H. Dammann, published in Optik 31, 1970, pp. 95-104, it is possible to approximate a continuous phase profile with a stepwise profile of discrete phase levels. Dammann shows that this approximation produces several diffraction orders where the phase of the first order matches exactly the phase of the continuous profile and its efficiency approaches 100% as more steps are added. For example, 10 steps produce a diffraction efficiency of the first order of almost 97%.
Etching or thin film coating techniques enable the production of the necessary multilevel profile. This is discussed in the following articles by Swanson and Veldkamp:
"Diffractive Optical Elements for Use in Infrared Systems," Optical Engineering, June 1989, Vol. 28 No. 6, pp. 605-608; and
"Infrared Applications of Diffractive Optical Elements,", SPIE Vol. 883: Holographic Optics: Design and Applications (1988), pp. 155-162, wherein the diffractive optical elements produced by their method have 2.sup.N steps produced via the use of N masks in N serial manufacturing cycles. The etching depths of each of the N steps are related by a fixed ratio and at each manufacturing cycle, each step is divided into two steps such that the number of steps is doubled.
As is known in the art, the width of each step becomes narrower as the number of steps increases and the minimum line width is typically determined by manufacturing constraints such as thinnest etching or deposition width. The method of Swanson and Veldkamp, restricts the number of steps that are produced to be a power of 2. Thus, if, for example, eight steps do not satisfy the efficiency requirements, sixteen steps must be produced. If manufacturing constraints do not permit sixteen steps to be produced, then only eight steps can be produced resulting in an optical element of lower efficiency than desired. If, however, manufacturing constraints do permit a number of steps between eight and sixteen, the abovementioned method does not provide a way to produce it.